The present invention relates to a method and to a device for evaluating the absolute permeability of a zone of an underground hydrocarbon reservoir from rock samples taken from this zone, such as cuttings obtained during well drilling operations.
The current petroleum context leads operators to taking an interest in new zones (deep offshore) and in new types of reservoirs (marginal structures close to existing surface installations). Considering the drilling costs linked with the difficult environment of these new discoveries or with the limited size of certain structures, operators can no longer allow themselves to drill complementary appraisal wells without taking the risk of compromising the economic viability of the project. The development strategy set before starting production is therefore less strict so as to allow  less than  less than real time greater than  greater than  adaptation to the nature of the information collected as a result of production well drilling, which is referred to as appraisal development.
Petrophysical measurements play a key part in the appraisal of the quality of a reservoir. However, the delays linked with this type of measurement are often very long and thus incompatible with the reactivity required for the success of such appraisal developments. New, faster and less expensive evaluation means are therefore sought as a decision-making support.
The cuttings carried along by the mud have been subjected to in-situ examinations for a long time. They are carried out by the teams in charge of mud logging operations and they are essentially intended to complete the description of the geologic layers crossed through during drilling, which is performed from logs.
Work has already been done to evaluate petrophysical properties from cuttings. Acoustic properties relative to S and P waves have been measured for example. Various parameters have also been studied, such as the hardness and the deformation of rock fragments, or the porosity and the permeability thereof.
According to a first known permeability measurement method, the rock fragment is previously coated with resin. A thin slice is cut from the coated rock and placed in a measuring cell. It comprises means for injecting a fluid under pressure at a controlled flow rate and means for measuring the pressure drop created by the sample. Since the resin is impermeable, the absolute permeability is deduced from Darcy""s equation by taking into account the real surface area occupied by the cuttings.
This method is for example described by:
Santarelli F. J. et al;  less than  less than Formation evaluation from logging on cuttings greater than  greater than , SPERE, June 1998, or
Marsala A. F. et al;  less than  less than Transient Method Implemented under Unsteady State Conditions for Low and Very Low Permeability Measurements on Cuttings greater than  greater than ; SPE/ISRM No.47202, Trondheim, Jul. 8-10, 1998.
This type of measurement can only be obtained in the laboratory after long cuttings conditioning operations.
Another method is based on an NMR (Nuclear Magnetic Resonance) measurement that is performed directly on the cuttings after previous washing followed by brine saturation. This type of measurement gives a directly exploitable porosity value. Permeability K is determined by means of correlations of the same nature as those used within the scope of NMR logging.
An illustration of this method can be found in the following document:
Nigh E. et al; P-K(trademark):  less than  less than Wellsite Determination of Porosity and Permeability Using Drilling Cuttings greater than  greater than , CWLS Journal, Vol.13, No.1, December 1984.
The object of the method according to the invention is to evaluate physical parameters such as the absolute permeability of porous rocks of an underground reservoir zone from rock fragments (cuttings for example) taken from this zone.
The method comprises:
immersing the fragments in a viscous fluid contained in a containment vessel,
a stage of injection, into the vessel, of the viscous fluid under a pressure that increases with time, up to a determined pressure threshold, so as to compress the gas trapped in the pores of the rock,
a relaxation stage after injection stop,
measuring the evolution of the pressure in the vessel during the two injection and relaxation stages,
modelling the evolution of the pressure during the injection and relaxation process, from initial values selected for the physical parameters of the fragments, and
a stage of iterative adjustment of the physical parameter values of the rock fragments so that the modelled evolution is best adjusted to the measured pressure evolution in the vessel.
According to the circumstances, the containment vessel can be filled with cuttings invaded by drilling fluids or previously cleaned.
The device according to the invention allows to evaluate physical parameters such as the absolute permeability of porous rocks of an underground reservoir zone, from rock fragments taken from this zone. It essentially comprises:
a containment vessel for porous rock fragments,
means for injecting a viscous fluid into the vessel in order first to fill the vessel containing the rock fragments, and to perform a cycle comprising a stage of injection, into the vessel, of fluid under a pressure that increases with time (preferably at a constant flow rate to facilitate measurement of the volume of fluid injected), up to a determined pressure threshold, then to compress the gas trapped in the pores of the rock, and a relaxation stage after injection stop,
means for measuring the evolution of the pressure in the vessel during the two injection and relaxation stages, and
a processing system for modelling the evolution of the pressure during the injection and relaxation process, from initial values selected for the physical parameters of the rock fragments, and for iteratively adjusting the values to be given to these physical parameters so that the modelled pressure evolution is best adjusted to the measured pressure evolution in the vessel.
The injection means comprise for example a pump injecting water at a constant flow rate into a surge tank filled with a high-viscosity oil communicating with the containment vessel through valves.
The method is satisfactory for rocks of very different permeabilities ranging from some millidarcy to several hundred millidarcy. Considering the limited surface area occupied by the implementation device and the speed with which the measurements and the adjustment between the theoretical data and the experimental data can be performed, the method lends itself particularly well to field conditions. It is thus quite possible to envisage measurement and interpretation directly on the site within a very short time, therefore with no possible comparison with those required to obtain equivalent results by means of laboratory methods. This opens up interesting possibilities as regards characterization since this new source of information can be put to good use as a support for interpretation of electric logs and to fine down evaluation of a well in terms of production potential.